This is a randomized, double-blind clinical trial involving subjects with prolonged prior experience with ZDV/ddI or ZDV/ddC combination therapies. Eligible subjects are participants in ACTG 175 who remanied of their initially assigned combination therapy until the end of follow-up in ACTG 175, or were crossed over to one of the combination therapies (because of a 50% deline for baseline in CD4+ cell count or the development of major HIV-related disease). They must also have remained on that combination therapy for any intervening period between the end of data collection for ACTG 175 (Feb 28,1995) and entry into ACTG 303. Subjects will be randomized to one of three treatment arms: eith to continue their ZDV/ddI or ZDV/ddC, or to add 3TC to their ZDV/ddI or ZDV/ddC, or to switch to ZDV/3TC. The study will be partially blinded in that subjects will take either ddI or ddI placebo, or ddC or ddC placebo, as well as 3TC placebo. The primary outcome measure is change in HIV RNA copy number. The duration of study treatment is planned to be 48 weeks.